Organic Light Emitting Diodes (OLEDs) are luminescent devices comprising organic layers, such as polymers. Each of these layers serves a specific purpose. In the past, a lot of effort was directed to optimization of the properties of the materials forming the structure. Several colors have been obtained, for example layers capable of producing red, green, blue, or white (which is in some OLEDs obtained by stacking three layers of the RGB materials) emission. When an appropriate combination of layers and emitters is used one can make these devices generate light with almost 100% internal efficiency. Even though light can be generated which such high efficiencies only a relatively small portion can be extracted from the structure. Due to total internal reflection (TIR) at surfaces with media having a lower refractive index and coupling to plasmonic modes, a large portion of the light remains trapped inside the stack and is eventually absorbed. Thus only a small percentage of light trespasses the structure resulting in an overall efficiency typically reaching values as low as 20%.
Some efforts have been made towards improving the outcoupling, for example improving the transparency and purity of the layers of conductive oxides (which are necessary to create carriers within the organic layers).
Other improvements for the outcoupling efficiency of the substrate also have been obtained by using micro lenses or patterning, hence reducing effects like total internal reflection at the substrate. The outcoupling is then limited by the quality of the substrate patterning, and the costs may increase while negatively affecting certain aspects of the image (for example reducing the viewing angle).
Document WO2007033490 discloses a layer comprising a transparent conductive film with columnar microstructures for obtaining birefringence, improving the efficiency of LCDs. This document may be applied to OLEDs, because the contrast and image stability of displays can be improved, but the outcoupling, although it may be improved in some cases, is difficult to control, the production is costly, complex and difficult to implement in OLEDs, as thin structures are needed.
Another effect that has been studied for optimization of the outcoupling efficiency is the orientation of the emitting elements. In “Optical design for efficient light emission in OLEDs and anisotropic layers”, SPIE Organic+ Electronics International Society for Optics and Photonics 2013, Penninck and Neyts describe the effect of the orientation of the emitting center behaving as electrical dipole antennas with respect to the substrate, on the outcoupling efficiency. It was found that when the dipole moment of the luminescent transition was parallel with the substrate, a higher outcoupling efficiency was obtained.
Nevertheless, there is still room for improvement of the outcoupling efficiency of OLED devices.